The thing to be careful of is whether a mainstream provider covers replacement of insulin & other diabetes stuff. Many will cover you with diabetes, often without any loading, but most will exclude any diabetes related issues from your cover, including replacement of lost or stolen medication.
I have used Flexicover for the last few years. I've shopped around & they're not the cheapest out there, but they cover more "dangerous" activities at a higher level. e.g. I waterski regularly on holiday, most companies only cover "incidental" waterskiing, I also go white water rafting & many again only cover up to grade 2 rapids, I'll raft on grade 3 & 4.
They are a specialist medical condition provider. You pay a basic fee for cover that accepts you as a diabetic but doesn't include cover for diabetes & a separate medical fee to cover the diabetes side of things (which adds about 100% to the cost in my case).
I am 43, have annual worldwide cover including USA & winter sports & it cost about £140 all in last year (I'm due to renew soon).
I travel frequently & to some unusual destinations, so cover for my insulin is vital.
Hope this helps.